I have a variables.cpp, in which it defines:

static MyNamespace::MyClass myObj('input');

I have a variables.h that is included by variables.cpp

And I have a main.cpp that includes variables.h, which it refers to myObj:

MyNameSpace::myObj.doRun();

And the compiler complains that: no member named 'myObj' in namespace 'MyNameSpace'

What am I doing wrong?

CodePudding user response：

The very purpose of static variables is to be file scope, not visible in other files. So naturally they are not then visible, using static specifically requests that.

As a consequence, several .cpp files can have a static variable with same name, and they will be different variables. Also as a consequence, if you define a static variable in .h file, then every .cpp file which includes it will have a different variable.

Extra notes:

• In C you can achieve similar effect (file scope) by putting the variables in anonymous namespace in the .cpp file.

• static class member variables are a different case, they are visible everywhere (in other words, the variable symbol name is exported), and basically behave like non-static global variables which aren't member variables. Yes, it can be a bit confusing, static has several different meanings in C .

• static variable can be accessed from other .cpp files, if you create a function which returns a reference (or a pointer) to it in the .cpp file where it is defined. It's lifetime is same as program execution, so this is safe from that point of view. However, thread safety and re-entrancy are then issues which need attention, so you shouldn't do this unless you know why you are doing this.

CodePudding user response：

as another answer point out, static variable is meant to not shared with others.

If you do want to share myObj, you should remove the static specifier

// variables.cpp
MyNamespace::MyClass myObj('input'); // no static

and declare it in header file

// variables.h
extern MyNamespace::MyClass myObj;

or, with c 17, another option is use inline variable.

// variables.h
inline MyNamespace::MyClass myObj('input');

// variables.cpp
// do not redeclare it 

btw, 'input' is multicharacter literal, most likely not what you want.